Birth: We basically show up in the world microbe-free, although some studies challenge just how sterile we really are. Regardless, things change quickly. Microbes are out there waiting to "colonize" on — and in — us as soon as we're born.

How and where we greet the world makes a difference. Vaginal births add bacteria that help the baby successfully digest its first meal. C-section babies pick up skin microbes. And kiddos born at home pick up different microbes from those born in hospitals.

One day — six months:Family and environment add to the microbe brew, which then begin to "specialize." The ones on the skin are different from those in the baby's mouth. Things are happening in the tummy, too. Breast-fed babies have different gut microbes from formula-fed babies, and solid foods trigger still other types.

Three years to adulthood: The microbiome stabilizes but changes in response to illness, antibiotics, stress and diet. Big turning points in life — puberty, pregnancy, menopause — result in more shifts.

Old age: The number of microbe species decreases. Here's what we're finding out: The implications don't end with basic health. There is growing evidence that the microbiome affects the nervous system and influences behavior. In fact, some experts suspect there's a brain link which could result in new ways of treating some conditions, like autism and depression.